"This was just -- blood all over the place," said Derek Osborne, who lives nearby.

Osborne said the violent scene unfolded just outside of his apartment.

Scene near 37th and Oklahoma

Derek Osborne

"The door was covered in blood. Just a mess," said Osborne.

He made the gruesome discovery moments after hearing an argument.

"About 4 p.m. to 5 p.m., I heard people arguing in the hallway and the door slammed," said Osborne.

That was followed by two gunshots. Milwaukee police said a 21-year-old man died at the scene.

Scene near 37th and Oklahoma

Scene near 37th and Oklahoma

Brittany Mosconi

"It's very heartbreaking. I'm just shocked. Just shocked. It makes me so sad," said Brittany Mosconi, a friend of the victim.

Mosconi said she can't understand why this happened.

"He kind of seemed shy and soft-spoken. You could tell he was a really good kid," said Mosconi.

"It's scary. If he would have shot and it went through somebody else's apartment, you never know what's going to happen. This is usually a pretty good place to live. We never usually have problems like that and it's just like, wow," said Osborne.

Police took one person into custody.

Meanwhile, Mosconi said this death was a tragic reminder to value her loved ones.

"I literally...it made me hold my son so much closer last night. I was like, how does somebody do that? I just hope he does get justice," said Mosconi.

FOX6 knows the victim's identity and will release that information pending police approval.